This invention relates in general to digital radiography, and in particular to the imaging of a long human body part, such as the spine or legs, using a storage phosphor-based computed radiography system.
When a long segment of the human body is imaged using the conventional screen-film technique, special cassettes and films of extended length are used, such as 14xe2x80x3xc3x9736xe2x80x3 and 14xe2x80x3xc3x9751xe2x80x3. As medical institutions are migrating from analog screen-film systems to digital modalities, such as computed radiography (CR), these types of exams impose a significant challenge. This is because the size of digital detector is limited. For example, the largest CR storage phosphor cassette from several major CR vendors is limited to 14xe2x80x3xc3x9717xe2x80x3, which can only image a portion of the long body part at a time.
Several methods have been proposed to extend the CR imaging coverage by stacking several existing standard CR cassettes. European Patent Application 0919856A1 (also U.S. Pat. No. 6,273,606B1) discloses a way of overlapping several storage phosphor cassettes 101 adjacently. The cassettes can be in the alternating (FIG. 1A), staircase-wise (FIG. 1B), or oblique (FIG. 1C) arrangement. During the x-ray exposure, all the partially overlapping cassettes are exposed simultaneously, therefore each storage phosphor screen 102 that resides inside the corresponding cassette records a part of the image of the long body part. Similar approach is disclosed by Japanese Patent Application 2000250153A. European Patent Application 0866342A1 (also U.S. Pat. No. 5,986,279A) presents a method that is based on partially overlapping a plurality of storage phosphor screens for extended imaging coverage. The screens 102 can also be configured in the alternating (FIG. 1D), staircase-wise (FIG. 1E), or oblique (FIG. 1F) overlapping arrangement in a single elongated cassette 103. Similar approaches are proposed in Japanese Patent Application 2000267210A and 2000241920A. Further, cassettes and screens can be used together in alternating arrangement as shown in FIG. 1G, the cassettes 101 and storage phosphor screens 102 are placed in a partially overlapping and alternating arrangement with the screens 102 always positioned in front of the cassettes 101. This method eliminates the cassette shadow from the acquired images, and reduces the number of storage phosphor screens that need to be removed out of and to be replaced back into cassettes.
Because the phosphor screens are the fundamental imaging recording devices, no matter whether the screens are packaged within the individual cassette or not, the term xe2x80x9cstorage phosphor screenxe2x80x9d, xe2x80x9cphosphor screenxe2x80x9d, or xe2x80x9cscreenxe2x80x9d is used hereinafter to represent either the phosphor screen itself or the phosphor screen that is conveyed inside a cassette. Therefore the different scenarios in FIG. 1 are reduced to alternating, staircase-wise, and oblique arrangements of overlapping phosphor screens, with the exception that the distance between the screen planes can vary for each scenario depending on if the screen(s) is conveyed inside the cassette or not.
A schematic view of how a patient radiographic image is acquired is shown in FIG. 2. The patient (element 203) is positioned between the x-ray source (element 201) and a plurality of screens (element 205). Any of the screen arrangement methods shown in FIGS. 1A-1G can be used for imaging. An optional anti-scatter grid (element 204) can be placed between the patient and the screens. The grid can be either a stationary type or reciprocating type. During x-ray exposure, the x-rays can be collimated to minimize the radiation to the nondiagnostically relevant patient anatomy. After the x-ray generator is fired and the cassette is exposed, the image of the patient is recorded by the plurality of screens as latent radiographic signals. Each screen captures only a portion of the image of the patient. The screens are fed into a CR reader and the latent radiographic signals are converted to electronic images. The electronic image acquired from an individual screen will be referred to as a sub-image.
The sub-images acquired by the individual storage phosphor screens must be stitched together to create a composite full image. Information about the spatial order, orientation, and overlap arrangement of the phosphor screens used during x-ray exposure is required in order to stitch together the composite image. After x-ray exposure, the phosphor screens may be scanned in an arbitrary order in the CR reader. It is therefore necessary to rearrange the sequence of the scanned sub-images into the order corresponding to the physical setup used for image acquisition. It is also required that the overlap arrangement between consecutive screens be known exactly. For example, screens can overlap either on the top or on the bottom in cases of staircase-wise and oblique screen arrangements, and in the case of alternating screen arrangement, screens closer to the x-ray source overlap differently from those further away. A screen may be equivalently positioned for image capture in the landscape (horizontal) orientation with either of the two long dimensions facing up. Consequently, a scanned sub-image may be rotated 180xc2x0 from an adjacent sub-image. Detection of sub-image rotation is therefore necessary.
U.S. Pat. Nos. 4,613,983 and 5,833,607 each disclose methods to reconstruct a composite radiographic image from a set of sub-images. The former method is predicated on all sub-images being sequentially acquired and scanned in a predetermined spatial position and orientation. The latter method relies on a hardware position sensor to determine the relative position of each sub-image during acquisition. Neither of these two methods can be applied to the situation when the scanned sub-image sequence does not match the sequence used during acquisition. European Patent Application 0919858A1 (also U.S. Pat. No. 6,269,177B1) proposes a stitching method that utilizes a pattern of reference markers for the alignment of sub-images. However, this patent does not teach how the sub-images are ordered and requires that the sub-images are properly oriented prior to the stitching operation. Another stitching method was proposed by Wei et al. (xe2x80x9cA new fully automatic method for CR image composition by white band detection and consistency rechecking,xe2x80x9d Guo-Qing Wei, et al., Proceedings of SPIE Medical Imaging, 2001, vol. 4322, pp 1570-1573), to automatically stitch sub-images read from phosphor sheets in staircase-wise arrangement. However, this method assumes the sub-images are already pair-wise sequentially arranged before stitching.
Japanese Patent Application 2000258861A discloses a method that depends upon two different identification labels attached on every phosphor screen as auxiliary information for determining the orientation and location of the corresponding screen. Japanese Patent Application 2000232976A further teaches how the auxiliary information can be used in the stitching process. However, it is desirable not to use auxiliary information at all because doing so usually means either the standard cassette needs to be modified or the standard phosphor screen IDs need to be replaced, both of which may cause the cassette or screen incompatible for other general purposes.
It is therefore desirable to develop an automatic method to determine the spatial order, orientation, and overlap arrangement of the phosphor screens used in the x-ray exposure directly based on the acquired sub-images.
According to the present invention, there is provided a solution to the problems discussed above.
According to a feature of the present invention, there is provided a method of a method for automatic arrangement determination of partial radiation images for reconstructing a stitched full image comprising: acquiring at least two radiation sub-images of an elongated object by means of corresponding overlapped radiation recording media; converting said recorded sub-images to digital images and storing them; constructing a hypothesis list that includes all possible yet unique arrangements of sub-image order, orientation and overlap; detecting overlap regions for each consecutive sub-images pair in each hypothesis; conducting a plurality of different measurements on every consecutive sub-image pair of each hypothesis and deleting the hypothesis from the list if any measurement result is out of range; calculating the correlation function of the overlap regions for each consecutive sub-image pair in each hypothesis left in said hypothesis list and finding the function maximum and the horizontal displacement between the sub-image pair; checking the magnitude of the horizontal displacement between each consecutive sub-image pair and rejecting the consecutive sub-image pair and rejecting the hypothesis form the hypothesis list if the magnitude is out of range; establishing an overall figure-of-merit for each hypothesis left in the hypothesis list based on the sum of the maximum of the correlation functions; selecting the hypothesis of maximum figure-of-merit for best candidate; and processing the sub-images based on selected hypothesis for stitching to produce an output image consisting of the stitched sub-images.
The invention has the following advantages:
1. This invention allows automatic determination of the spatial order, orientation, and overlap arrangement of the phosphor screens used in the x-ray exposure directly based on the acquired sub-images.
2. The method allows phosphor screens to overlap arbitrarily, to be positioned with a 180xc2x0 variability, and to be scanned in the CR reader in any order.
3. When the invention is combined with an automatic image stitching method, the entire process of producing a composite image from a set of sub-images can be made fully automatic.
FIGS. 1A-1C are diagrammatic views showing a plurality of storage phosphor cassettes, with each cassette containing one storage phosphor screen, arranged in alternating, staircase-wise, and oblique positions, respectively. The storage phosphor screens are represented as solid vertical lines inside the cassettes.
FIGS. 1D-1F are diagrammatic views showing a plurality of storage phosphor screens arranged in alternating, staircase-wise, and oblique positions, respectively. The screens can be contained within a single, extended length cassette.
FIG. 1G is a diagrammatic view showing a configuration consisting of a set of storage phosphor screens/cassettes that are placed in an alternating arrangement with the screens placed in front of the cassettes (closer to x-ray source).
FIG. 2 is a diagrammatic view showing a method for image acquisition using the alternating phosphor screen/cassette configuration shown in FIG. 1G as an example. Any of the other configurations shown in FIG. 1 can also be used for acquiring the images.
FIG. 3A is a diagrammatic view showing three sub-images acquired using the configuration shown in FIG. 1G. The middle image 302 is recorded on a storage phosphor screen (either the screen itself or the screen within a cassette) that is placed closer to the x-ray source. The top image 301 and the bottom image 303 are recorded on two storage phosphor screens (either the screen itself or the screen within a cassette) that are placed behind the middle screen. The shadow of the middle screen top edge is recorded in image 301, and the shadow of the bottom-edge is recorded in image 303.
FIG. 3B is a diagrammatic view showing a modification of the middle image of FIG. 3A.
FIGS. 4A and 4B are diagrammatic views illustrating the definitions of the front screen, back screen, front screen overlap edge, and back screen overlap edge.
FIG. 5 is a flow diagram showing an embodiment of the method of the present invention.
FIG. 6 is a diagrammatic view illustrating the major image processing steps that are used to automatically find the locations and orientations of the screen overlap edges in both the front and the back images, and for finding the location and orientation of the shadow of the front screen overlap edge in the back image.
FIG. 7 is a diagrammatic view illustrating the major image processing steps that are used for calculating the correlation function for the overlap regions between a consecutive sub-image pair.
FIG. 8 lists all the hypotheses that include different spatial order, orientation, and overlap arrangements of the sub-images when three screens are used in alternating arrangement.
FIG. 9 shows the screen attenuation level as a function of x-ray beam energy.